1. Field of the Invention
The present invention relates to a display device, in particular, a display device including thin-film transistors formed on a substrate.
2. Description of the Related Art
A so-called active-matrix display device includes a plurality of pixels arranged in matrix in a display area of a substrate. A thin-film transistor is formed in each of the plurality of pixels. The thin-film transistors are turned ON by scanning signals fed to gate signal lines, each being connected commonly to a group of pixels arranged in a row direction, thereby selecting the groups of pixels. Then, video signals fed through drain signal lines, each being connected commonly to a group of pixels arranged in a column direction, are respectively input to the pixels included in the selected groups of pixels through the thin-film transistors in an ON state, thereby driving the pixels.
The display device may include a scanning-signal drive circuit and a video-signal drive circuit that are formed in the periphery of the display area of the same substrate. The scanning-signal drive circuit feeds the scanning signals to the gate signal lines, whereas the video-signal drive circuit feeds the video signals to the drain signal lines. Each of the scanning-signal drive circuit and the video-signal drive circuit includes a plurality of thin-film transistors that are formed at the same time when the thin-film transistors are formed in the pixels.
In this case, some of the thin-film transistors include a crystalline semiconductor layer made of polysilicon or the like as a semiconductor layer, whereas the other thin-film transistors include an amorphous semiconductor layer made of amorphous silicon or the like as the semiconductor layer.
In recent years, however, a thin-film transistor having the following structure, for example, as disclosed in Japanese Patent Application Laid-open No. 2005-167051 is known. The thin-film transistor includes a source layer and a drain layer formed on a gate insulating film, which is formed so as to cover a gate electrode, through an intermediation of a channel layer. As the channel layer, a channel layer formed of a laminate of the amorphous semiconductor layer and the crystalline semiconductor layer which is provided between the amorphous semiconductor layer and the gate insulating film is known. The thin-film transistor having the structure described above has effects of reducing a so-called S value (swing factor) corresponding to one of initial characteristics and keeping a variation in threshold voltage with time small.
In the thin-film transistor disclosed in Japanese Patent Application Laid-open No. 2005-167051, however, the laminate of the amorphous semiconductor layer and the crystalline semiconductor layer is formed in an island shape. Each of a source electrode formed on the source layer and a drain electrode formed on the drain layer is formed so as to extend along a side wall surface of the laminate so as to be superimposed on the gate insulating film.
In such a structure, the source electrode and the drain layer are in contact with each other on the side wall surfaces of the crystalline semiconductor layer. As a result, a so-called off-leak current is disadvantageously increased.
When the thin-film transistor having the above-mentioned structure is used as a transistor for pixel selection, there arises a problem that contrast of the display area is prevented from being improved.